When a car turns, the outside wheels of the turn must rotate faster and travel at a greater distance than the inside wheels. The conventional differential gearing units allow this to happen. Cars can easily become stuck in slippery conditions such as mud, snow or sand, because of a lack of friction on the rotating wheel even if the other three wheel have good traction.
The conventional differential gearing unit in a two-wheel drive system would only rotate one wheel when the car gets stuck, since the conventional gearing units do not lock two wheel together. Similarly, in a four-wheel drive system, the conventional gearing units would only rotate two wheels when the cars get stuck, because four wheels are not locked together.
In the conventional four-wheel drive mechanism, as shown in FIG. 1, there are two drive shafts. Number 1 is for the front axle, and number 2 is for the rear axle. These are connected to two conventional differential gearing units, shaft 1, and shaft 2 have same ratio of rotation which means the total rotations of two front wheels must be equal to the total rotation of two rear wheels. The same ratio of rotations is only good when the car moves on a straight line with four wheels of the same size. However, when the car turns, the total number of rotations of the two front wheels are larger than the total number of rotations of the two rear wheels.
The result is one or more of the wheels will skid on the pavement; therefore, the engine has to try harder to overcome this skidding. The same situation will occur when the car moves on a straight line with one wheel smaller than the others (for example, when the smaller spare tire is used or when a tire has low air). Thus, energy is wasted and tires are easily worn out.